=====утверждения lookahead=====

Регулярное выражение:
<code>
^(?=(?:\D*\d){0,3}\D*$).{6,25}$
</code>

Объяснение: 
<code php>
^           # Start of string
(?=         # Assert that the following can be matched here:
 (?:\D*\d)  # Any number of non-digits, followed by one digit
 {0,3}      # (zero to three times)
 \D*        # followed by only non-digits
 $          # until the end of the string
)           # (End of lookahead)
.{6,25}     # Match 6 to 25 characters (any characters except newlines)
$           # End of string
</code>

[[http://www.regular-expressions.info/lookaround.html|подробнее]]